People seem to confuse the PDI with the walk-thru.
When I worked in the industry, at an exceptionally reputable dealer with a stellar reputation it worked this way. By the way, the owner/manager of the dealership was a former Coast Guard Captain and the dealership was run like a ship. (As an aside, most dealers no longer do it this way.)
Every unit sold, in fact, had been through TWO PDI's. First, when the unit arrived from the manufacturer, I inspected the unit and wrote up the work orders from and extensive checklist. Warranty repairs were then made before the unit even hit the lot. Used/trade-in units were treated in like manner.
After a unit was sold, another PDI was performed from an even more extensive checklist. This covered systems, mechanicals, cosmetics, and any damage that may have occurred while the unit was on the lot. Once again work orders were written and defects were corrected. All deficient items were reinspected by someone (a manager) other than the technician who did the work. The units were then thoroughly cleaned.
As part of the delivery/walk-thru process, the PDI checklist was given to the customer including the list of deficiencies that were corrected so the customer could pay close attention to them in the walk-thru (and monitor their performance down the road). The customer was given a detailed explanation, and demonstration, of all systems and shown where hard to find items (like low point drains) were located. If they weren't labeled by the manufacturer, we labeled them for the customer.
The customer was not part of the PDI process. How could they since some issues may be corrected on the spot, during the PDI, and the PDI occurred in the shop where customers are not allowed per safety policies or regulations. Plus, you can't have a customer hovering over a service writer or technician, slowing them down and possibly getting in the way, while the PDI is being performed.